Early Dart Headliner Install Question

I'm prepping to install a new headliner in my 66 Dart hardtop. I've got all of the headliner bows (6 total) in the correct order and the two wire clips for the rear-most headliner bow.

In reviewing the FSM, it mentions a "drive nail" in Dart models that attach the #5 listing to the quarter panel. Now my 66 Dart's been apart for years now and I don't remember anything about this so-called drive nail. Does anyone have any idea what the FSM is referring to? Better yet, does anyone happen to have a photo of said drive nail installed? I'm wondering how necessary this thing is. I don't understand what it would even do.

Here are the pertinent screenshots from the '66 FSM:

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